THE RELATION OF PLANTS TO WATER 



149 



dry season the amount of available water in the sandy or alkaline 

 soil is very small, and the roots, which often extend to great 

 depths, are wholly unable to provide a large volume of water 

 for the aerial stem and leaves. The atmosphere also, in these 

 regions, is dry and hot during the long, dry season, and the plant 

 is thus in danger of losing the small amount of water available 

 from the roots. Under these extreme conditions all plants of the 

 expanded mesophytic type are likely to be destroyed, and only 



FIG. 72. Leaves of xerophytes protected by hairs from excessive 

 loss of water 



a, hairs of wormwood ; b, of Convolvulus; c, of Elaeagnus. After Korner 



those contracted xerophytic forms survive which are adapted to 

 the peculiar conditions existing in the desert. These xerophytic 

 desert plants are characterized not only by their contracted form 

 and restricted leaf surface but by peculiarities in their structure 

 as well. The protective epidermis in such plants is wont to be 

 coated with wax or cutin, or is supplied with abundant hairs to 

 protect the plant from excessive surface evaporation. The inter- 

 nal tissues are also more compact, with fewer and smaller inter- 

 cellular spaces into which water vapor can pass from the living 

 cells and then into the external atmosphere. Many plants of this 

 character are known also to have dense cell sap, which enables 

 them to hold water vapor and thus prevent its loss by evapora- 

 tion. These and various other modifications looking toward the 



